1. Field of the Invention
The present invention relates to boilers and, in particular, to a method and apparatus for measuring the effectiveness of sootblowers that remove ash deposits on the superheaters of the boilers used with the kraft pulping process.
2. Background Information
In the paper-making process, chemical pulping yields, as a by-product, black liquor, which contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a boiler. The two main functions of the boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. The twin objectives of recovering both chemicals and energy make boiler design and operation very complex. As used herein, a “boiler” indicates a top supported boiler that, as described below, burns a fuel which fouls the heat transfer surfaces.
In a kraft boiler, superheaters are placed in the upper furnace in order to extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section, and superheated steam exits at a controlled temperature. The superheater is constructed of an array of tube panels. The superheater surface is continually being fouled by ash that is being carried out of the furnace chamber. The amount of black liquor that can be burned in a kraft boiler is often limited by the rate and extent of fouling on the surfaces of the superheater. This fouling with deposited ash reduces the heat absorbed from the liquor combustion, resulting in low exit steam temperatures from the superheaters and high gas temperatures entering the boiler. Boiler shutdown for cleaning is required when either the exit steam temperature is too low for use in downstream equipment or the temperature entering the boiler bank exceeds the melting temperature of the deposits, resulting in gas side pluggage of the boiler bank. In addition, eventually fouling causes plugging, and in order to remove the plugging, the burning process in the boiler has to be stopped. A plugged boiler typically means at least a twenty-four hour shutdown for the entire production unit, which causes great economic losses for the entire pulp mill. Kraft boilers are particularly prone to the problem of superheater fouling, due to the high quantity of ash in the fuel (typically more than 35%) and the low melting temperature of the ash.
There are three conventional methods of removing ash deposits from the superheaters in kraft boilers, listed in increasing order of required downtime and decreasing order of frequency: 1) sootblowing; 2) chill-and-blow; and 3) waterwashing. This application addresses only the first of these methods, sootblowing. Sootblowing is the process of blowing deposited ashes off the superheater with a blast of steam from nozzles called sootblowers. Sootblowing occurs essentially continuously during normal boiler operation, with different sootblowers turned on at different times. Sootblowing is usually carried out using steam, the steam consumption of a sootblowing procedure typically being 4-5 kg/s, which corresponds to about 4-5% of the steam production of the entire boiler; the sootblowing procedure thus consumes a large amount of thermal energy.
At its simplest, sootblowing is a procedure known as sequence sootblowing, wherein sootblowers operate at determined intervals in an order determined by a certain predetermined list. The sootblowing procedure runs at its own pace according to the list, irrespective of whether sootblowing is needed or not, which means that plugging cannot necessarily be prevented even if the sootblowing procedure consumes a high amount of steam. Each sootblowing operation reduces a portion of the nearby ash deposit, but the ash deposit nevertheless continues to build up over time. As the deposit grows, sootblowing becomes gradually less effective and results in impairment of the heat transfer. When the ash deposit reaches a certain threshold where boiler efficiency is significantly reduced and sootblowing is insufficiently effective, deposits may need to be removed by one of the other cleaning processes identified above.
A steam sootblower is, typically, elongated tubes having one or more radial openings at the distal end. The tubes are coupled to a source for pressurized steam. The sootblowers are further structured to move between a first position located outside of the furnace, to a second location within the furnace. As the sootblowers move between the first position and the second position, the sootblower moves adjacent to the heat transfer surfaces. One type of sootblower is structured to move generally perpendicular to the heat transfer surfaces. Another type of sootblower moves generally parallel and in between heat transfer surfaces. To move perpendicular to the heat transfer surfaces, the heat transfer surfaces have passages therethrough. The movement into the furnace, which is typically the movement between the first and second positions, may be identified as a “first stroke” and the movement out of the furnace, which is typically the movement between the second position and the first position, may be identified as the “second stroke.” Generally, sootblowing methods use the full motion of the sootblower between the first position and the second position, however, a partial motion may also be considered a first or second stroke. As the sootblower moves adjacent to the heat transfer surfaces, the steam is expelled through the openings. The steam contacts the ash deposits on the heat transfer surfaces and dislodges a quantity of ash; some ash, however, remains. As used herein, the term “removed ash” shall refer to the ash deposit that is removed by the sootblowing procedure and “residual ash” shall refer to the ash that remains on a heat transfer surface after the sootblowing procedure. The steam is usually applied during both the first and second strokes.
Rather than simply running the sootblowers on a schedule, it is desirable to actuate the sootblowers when the ash buildup reaches a predetermined level. One method of determining the amount of buildup of ash on the heat transfer surfaces within the furnace is to measure the weight of the heat transfer surfaces and associated superheater components. The method of determining the weight of the deposits is disclosed in U.S. Pat. No. 6,323,442, which is incorporated herein by reference. It is further desirable to conserve energy by having the sootblowers use steam only when the steam is effectively cleaning the heat transfer surfaces.
There is, therefore, a need for a method of cleaning the heat transfer surfaces of furnace superheater components when the heat transfer surfaces attain a predetermined level of fouling.
There is a further need for a method of cleaning the heat transfer surfaces of furnace superheater components that only utilizes steam during an effective portion of the cleaning procedure.